Conventionally, in a communications system which employs mobile station terminals such as portable telephones, radio base station apparatuses (hereinafter called the “base station”) are provided for transmitting/receiving signals to/from mobile station terminals in order to enable communications with the mobile station terminals over the air within a coverage area which is an area in which communication services are provided. One base station is installed in every predetermined area such that a plurality of base stations cover the entire coverage area. An area covered by one base station is called a “cell.” A plurality of base stations are connected to a radio network controller which controls the base stations, and are connected to a communications network, which serves as a telephone network, through the radio network controller.
As a user manipulates a mobile station terminal within a coverage area, the mobile station terminal generally transmits/receives signals to/from the base station closest thereto, and is connected to a destination terminal through a communications network in a communicable manner. A signal transmitted by a base station to a mobile station terminal includes a common channel which has contents common to all mobile station terminals within a cell, and includes a dedicated channel whose contents differ from one mobile station terminal to another. The base station can adjust transmission power on a channel-by-channel basis.
The common channel includes information for operating a communications system such as cell information, and information required for a terminal to communicate with a base station. The cell information includes information required for a mobile station terminal to establish synchronization with a base station, information on neighboring cells, information on an interference level, a cell regulation state, and the like.
The dedicated channel is a channel for carrying user data which is data specific to an individual mobile station terminal, and for carrying data at high communication rates such as voice data, voice and image data for a videophone, packets and the like.
When multiple users attempt to manipulate their mobile station terminals for placing a call within a cell, communications of the multiple mobile station terminals concentrate on a single base station which covers the cell. Then, since the base station must transmit a signal of a different dedicated channel to each of the mobile station terminals which are in communication, the total transmission power of the base station is larger because the number of mobile station terminals that are used has increased. Also, even when there are not so many mobile station terminals, the total transmission power from the base station can increase if a poor radio propagation environment is present between the mobile station terminals and base station. To protect the base station apparatus from the total transmission power which increases in this manner, a limit value is set to a value which is lower than maximum transmission power which can be actually provided by the base station. In the following, the total transmission power reaching the limit value is assumed to be “100% of the total transmission power.”
The base station conducts a control of limit for adjusting the total transmission power to be equal to or lower than 100% in order to protect the apparatus, when the total transmission power exceeds 100%, by uniformly reducing the transmission power irrespective of the dedicated channel or common channel. An example of this control of limit is disclosed in JP-A-11-234203. The control of limit for the total transmission power in the base station will be described with reference to drawings.
FIGS. 1A and 1B are diagrams for describing the control of limit.
As shown in FIG. 1A, the total transmission power of the base station can exceed 100% due to an increase in the number of mobile station terminals connected to the base station for communication, or due to an exacerbated radio propagation environment between the base station and mobile station terminals. In this event, the apparatus itself will fail if the base station continues to provide the transmission power which exceeds the limit value.
FIG. 1B shows a manner in which the base station conducts the control of limit when the total transmission power of the base station exceeds 100%. When the control of limit is conducted, the base station reduces the transmission power of all channels by a fixed amount irrespective of the dedicated channel or common channel. Reference numeral 500 indicates the amount of attenuation.
A company which operates the communications system described above (hereinafter called the “system operation company”) may wish to give priority levels to certain calls which mean communication connection of the base station with mobile station terminals. For example, the system operation company may wish to give a higher priority level to urgent calls from mobile station terminals to the police or fire station in urgent cases.
Even if it is desired to give priority levels to calls in this way, the conventional limiter control reduces the transmission power for all mobile station terminals by a fixed amount without considering call priority levels, so that the reduction is also applied to the transmission power value for a mobile station terminal which places a highly prioritized call. For this reason, a highly prioritized call can be disconnected. A call is disconnected due to a reduction in the transmission power from the base station, which results in a failure to ensure a signal quality required for signals received by a mobile station terminal from the base station.
Also, since the transmission power of the common channel uniformly attenuates as well, the area covered by the base station is reduced. This can give rise to a failure in hand-over between base stations by a mobile station terminal, or the inability of a mobile station terminal to originate or receive signals even within a cell which can be essentially covered by the base station. In such a circumstance, services are not sufficiently provided to users of mobile terminals which place highly prioritized calls.